1. Field of the Invention
The present invention relates to a photo-electric converting device and its driving method, and its manufacturing method. Also, the present invention relates to a solid-state image pickup device, and its driving method and its manufacturing method.
2. Description of the Related Art
A solid-state image pickup device, for example, an ordinary solid-state image pickup device includes a plurality of photo-electric converting portions like photo-diodes arrayed as light-receiving cells in a one-dimensional fashion (linearly) or in a two-dimensional fashion (in a matrix fashion) on the surface of a semiconductor substrate. When light becomes incident on each of these photo-electric converting portions, signal charges are generated in response to the light and those signal charges are read out by a transfer electrode as a video signal.
FIG. 1 of the accompanying drawings is a cross-sectional view showing a solid-state image pickup device having such an arrangement according to the related art in an enlarged-scale. As shown in FIG. 1, a P-type first semiconductor well layer 52 is formed within an N-type silicon semiconductor substrate 51, and a photo-diode comprising an N-type semiconductor region (so-called electric charge accumulating region) 54 and a P+ semiconductor region (positive electric charge accumulating region) 55 formed on the N-type semiconductor region 54 is formed on the surface of the semiconductor substrate 51.
An N-type transfer channel region 57 comprising a vertical CCD (charge-coupled device) portion 56 is formed on one side of the row of the photo-electric converting portion 53, and a P-type second semiconductor well region 58 is formed under an N-type transfer channel region 57.
A pixel separating region for separating the adjacent pixels adjoining in the horizontal direction, that is, a P+ channel stopper region 59 is formed on the other side of the photo-electric converting portion 53. A transfer gate portion 60 is formed between the photo-electric converting portion 53 and the vertical CCD portion 56 to read signal charges from the photo-electric converting portion 53 to the vertical CCD portion 56.
A silicon oxide film 61 is formed on the surface of the semiconductor substrate 51, and a transfer electrode 62 formed of a poly-crystalline silicon layer is formed on the N-type transfer channel region 57 and the channel stopper region 49. Then, the transfer channel region 57 and the transfer electrode 62 constitute the vertical CCD portion 56. A light-shielding film 63 made of a metal material such as aluminum or tungsten is formed on this transfer electrode 62 through the silicon oxide film 61.
A cover film (passivation film) 64 formed of a transparent silicon oxide film (for example, PSG (phosphor-silicate glasses)) is formed on the whole surface of the light-shielding film 63. Then, a planarized film 65 is formed on this cover film 64 and an on-chip microlens 66 is formed on the planarized film 65 at its position opposing the photo-electric converting portion 53 through a color filter (though not shown).
In the solid-state image pickup device having the above arrangement, a loss of light that is introduced to the photo-electric converting portion 53 from the outside is caused to increase by reflection of light at the interface between the surface of the photo-electric converting portion 53 formed on the semiconductor substrate 51 and the silicon oxide film 61.
More specifically, since transmittance of light at the interface between the surface of the semiconductor substrate 51 and the silicon oxide film 61 is low, the amount of light that can be received by the photo-electric converting portion 53 decreases and hence the photo-electric converting portion 53 cannot obtain sufficient sensitivity.
To solve the above-mentioned problem, there is known a solid-state image pickup device having an arrangement in which an anti-reflection film is formed on the surface of the photo-electric converting portion 53.
More specifically, FIG. 2 is a cross-sectional view showing an example of such solid-state image pickup device having the above arrangement in an enlarged-scale. As shown in FIG. 2, an anti-reflection film 67 is composed of a silicon oxide film 68 formed on the whole surface of the photo-electric converting portion 53 and a silicon nitride film 69, for example, formed on the silicon oxide film 68 and which is made of a material having a refractive index higher than that of the silicon oxide film 68 and which is lower than that of the semiconductor substrate 51.
For example, a refractive index of the silicon oxide film 68 is 1.45 and a refractive index of the silicon nitride film 69 is 2.00. Also, a film thickness of the silicon oxide film 68 and a film thickness of the silicon nitride film 69 are selected to be less than 600 angstroms, preferably, they should be selected in a range of from 250 to 350 angstroms.
In FIG. 2, elements and parts identical to those of FIG. 1 are denoted by identical reference numerals.
Since the anti-reflection film 67 is formed by laminating the silicon oxide film 68 and the silicon nitride film 69 whose refractive indexes and film thicknesses are stipulated as described above, it becomes possible to decrease the reflectance of light on the surface of the photo-electric converting portion 53 up to about 12% to 13%. This means that the reflectance can be decreased up to approximately ⅓ as compared with the case of the solid-state image pickup device without the anti-reflection film 67 in which the reflectance of light on the surface of the photo-electric converting portion 53 is about 40% as shown in FIG. 1.
Although the anti-reflection film 67 has the silicon oxide film 68 interposed thereon to alleviate stress produced between the semiconductor substrate 51 and the silicon nitride film 69 of the anti-reflection film 67, since this silicon oxide film 68 is made of a material having a middle refractive index between that of single crystal silicon, which is a major material of the semiconductor substrate 51, and that of the silicon nitride film 69, the silicon oxide film 68 is interposed in the anti-reflection film 67, and hence reflection of light unavoidably occurs on the interface of the upper side or the lower side of the silicon oxide film 68.
As a consequence, the amount of light that becomes incident on the photo-electric converting portion 53 decreases finally, and a problem arises, in which the photo-electric converting portion 53 cannot obtain sufficient sensitivity.
In order to solve the above-mentioned problem, it is proposed to further decrease the film thickness of the silicon oxide film 68, for example. However, it does not matter how the film thickness of the silicon oxide film 68 should be decreased, so long as the silicon oxide film 68 is interposed in the anti-reflection film 67, it is unavoidable that reflection of light occurs on the interface of the upper side or the lower side of the silicon oxide film 68.
That is, since the refractive index changes intermittently in a series of light path of incident light such as the silicon nitride film 69, the silicon oxide film 68 and the surface of the semiconductor substrate 51, it is unavoidable that reflection occurs in the incident light at the interfaces of the respective layers.
In order to solve the above-mentioned problem, there is known an arrangement in which a photonic crystal is provided on a semiconductor substrate, for example (see cited patent reference 1).
More specifically, in this photonic crystal, when a distance (pitch) between the adjacent photonic crystals is selected to be approximately ½ of a wavelength of light that is set as an image pickup target, the refractive index gently changes from the upper portion of the interface of the silicon semiconductor substrate to the inside of the interface of the silicon semiconductor substrate. As a result, a loss of light can be suppressed as compared with the case in which light becomes incident on the interface of the flat semiconductor substrate, for example, and an amount of light that becomes incident on the photo-electric converting portion 53 can increase, thereby resulting in high sensitivity being obtained.
[Cited Patent Reference]
Official Gazette of Japanese laid-open patent application No. 2003-35846
However, when the photonic crystal is provided on the semiconductor substrate as described above, the following problem arises, for example.
More specifically, when the photonic crystal is formed, stress occurs in the single crystal silicon comprising the semiconductor substrate, a crystal defect occurs on the surface (interface) of the semiconductor substrate 1, and hence a dark current is increased by the interface states caused by the crystal defect.
Accordingly, although it becomes possible to obtain sufficient sensitivity by the photonic crystal provided on the semiconductor substrate, the increase of the dark current from the interface states cannot be suppressed and hence it is difficult to improve an S/N (signal-to-noise ratio), for example.
More specifically, when the photonic crystal is provided on the semiconductor substrate, it was difficult to achieve both the high sensitivity characteristic and the decrease of the dark current at the same time.
The above-mentioned problems occur not only in the solid-state image pickup device having the above arrangement but also in a photo-electric converting device of which photo-electric converting portion is formed of a photo-coupler used singly and the like.